European Patent EP-B 681672 describes a device which is designed for conveying thick matter containing shredded scrap metal. In that device, the thick matter is fed to the delivery device via the material feed space and is moved by the delivery piston into the delivery cylinder, from where it is forced through the delivery line. The closure member provides that, during each stroke, the thick matter is first compressed in the delivery cylinder before it is forced into the delivery line while the closure member is open. This device conveys the thick matter through the delivery conduit along with the foreign substances contained therein, including scrap metal and similar solids.
In principle, this device can also be used to convey biogenic wastes, which generally contain not only organic components, but also considerable amounts of foreign substances. The foreign substances must be removed from the material flow for a subsequent processing step. As long as the waste is in a relatively dry form, the separation of foreign substances is mostly done by hand. However, in the case of wet organic waste, the method chosen is that of shredding. In the treatment of food remains, for example, one uses hammer mills capable of shredding foreign substances, such as knife handles, to particles of less than 10 mm in size. Thick matter that is conditioned in this way can indeed be transported by a thick matter pump of the type specified at the outset. However, the disadvantage here is that the foreign substances are still contained in the material flow. The material separation problem is not solved in this manner, but shifted to a process step at the end of the transport process. This is the procedure used, for example, for food waste substrate which is fed to a fermenter and which still contains foreign substances such as shredded plastic packaging, cans, tubes, knives, forks. There, the organic matter is indeed degraded in the fermentation residue, but the shredded foreign substances contained in the sludge must still be removed if the sludge is intended for use as an organic nitrogen fertilizer. Without separation, the only possible method of disposal is by burning.